Walsh, who begins another string of dates with the Eagles on Tuesday in Pittsburgh, has been a regular presence alongside band leader Dave Grohl and company over the years. "You have to be in a good mood around those guys," Walsh once said of the Foo Fighters, in a talk with Rolling Stone. "It's wonderful energy."

The Foo Fighters' 20-song set in Boston also saw another guest turn by Dr. Lew Schon, the orthopedic surgeon who helped repair Grohl's broken leg in 2015. He initially joined them at Fenway that same year; they once again covered "Blitzkrieg Bop" by the Ramones.

Walsh and Grohl memorably performed "While My Guitar Gently Weeps" during the 2014 Grammy salute to the Beatles. They also collaborated on the new song "Outside" from the Foo Fighters' Sonic Highways album and HBO series. They co-starred in a video for the song, filmed at Rancho de la Luna in Joshua Tree, Calif. Walsh then sat in for a separate run through of "Rocky Mountain Way" and "Outside" at the Roxy in West Hollywood.